1. Field of the Invention
The present invention relates to a router device to be connected to Internet accommodating mobile terminals, and a datagram transfer method and a communication system using such a router device.
2. Description of the Background Art
The router device is used in connecting LANs (Local Area Networks) and plays a role of transferring datagram from one LAN to another LAN. Datagram contains communication information data to be transferred as well as a description of its source and destination network layer address (such as IP addresses in the case of IP), and the router device determines an output interface (port number) and a next transfer target node (router device or host functioning as communication terminal) of the datagram according to such an address information contained in the datagram.
Now, the routing by the router device in a network will be described for an exemplary case of transmitting an IP packet from a terminal A to a terminal B. In this case, the IP packet transmitted from the terminal A is routed through Internet according to its IP address and transmitted up to a subnet to which the terminal B belongs. Here, the IP packet transmitted from the terminal A is applied with the IP forwarding processing in a form of: datalink layer→IP packet→processing of IP header information and the like and output target determination→datalink layer packet, whenever the IP packet passes through a router device within Internet. Once the IP packet reaches to a router device in a final subnet, the logical-physical address conversion is carried out by ARP (Address Resolution Protocol) (by which the MAC address is obtained from the IP address), and the IP packet is transmitted to the terminal B after being converted into a datalink layer packet by attaching a datalink layer header information such as this MAC address.
Note here that, at a time of transferring datagram, the conventional router device realizes the processing up to the datalink layer (layer 2) by hardware and the processing of the network layer (such as IP forwarding processing, for example) by software.
However, such a conventional router device has been associated with a problem that the throughput of the software processing on the network layer is low compared with the fast hardware processing up to the datalink layer and causes a bottleneck so that the advantage of the fast hardware processing cannot be taken fully.
In order to resolve this problem, currently there are intensive research and development activities on a “high speed router device” for realizing the fast IP packet transfer.
The high speed router device can shorten the “delay time” since the packet is entered until it is outputted by using the hardware processing rather than the software processing for the packet transfer, and thereby realize as much faster transfer processing. The usual delay time due to the packet transfer processing by software is several milli-seconds, whereas the delay time of the packet transfer processing by hardware is about 100 micro-seconds, so that approximately ten times faster packet transfer can be realized.
There are two propositions for such a high speed router device, including the cut-through scheme and the hop-by-hop scheme.
(1) High speed router device in Cut-through scheme:
The cut-through scheme is a scheme in which the transfer processing is handled by a layer 2 switch on a lower level layer. Prior to the cut-through, the layer 2 information such as MAC address is exchanged by a specific protocol between terminals or router devices. Then, the router device realizes the transfer by setting up a cut-through path for by-passing through the layer 2 switch without bringing the processing up to the network layer (layer 3).
(2) High speed router device in Hop-by-hop scheme:
The hop-by-hop scheme is a scheme in which no exchange by a specific protocol as in the cut-through is required and the same processing as applied in the general router is applied to every input packet within the high speed router device. It differs from the general router in that the processing is done by the special hardware chip. The packet transfer processing to be carried out by the router goes through the processes such as CRC check of a frame, reading of a destination IP address within an IP packet, filtering processing, searching through a routing table, and replacement of MAC address, and the same processing is to be carried out every time except for the processing such as filtering. By utilizing the hardware processing or cache processing in such a portion of the processing which is to be carried out every time, it is possible to realize the fast transfer processing.
There is also a scheme in which the processing delay is suppressed by omitting the routing processing itself rather than utilizing hardware for the routing processing. Namely, in this scheme, an external switch node is provided in addition to the conventional router device, and this switch node has an ARP table in which IP addresses and MAC addresses of terminals moving over plural subnets are set in correspondence. When a packet destined to another subnet is entered from one subnet, this packet is directly sent to the destination terminal by looking up the ARP table. In this scheme, however, only the communications between subnets which are directly connected to the switch node are possible and the transfer processing for a subnet which is not directly connected cannot be done because there is no routing processing, so that there is a need to use the switch node and the usual router simultaneously.
Thus, currently there are intensive research and development activities on a “high speed router device” for realizing the fast IP packet transfer by resolving the bottleneck of the network layer processing in the router device. On the other hand, there are also research and development activities for a technique to accommodate mobile terminals in Internet type network. Such a mobile access technique includes a scheme using DHCP (Dynamical Host Configuration Protocol) server and a scheme using Mobile IP.
The scheme using DHCP server is a scheme in which the mobile terminal makes an Internet access by temporarily obtaining an IP address from the DHCP server within the network. The problem associated with this scheme using DHCP server is that the strategy to utilize the IP address dynamically obtained from the network of the visited site works well in the case where the mobile terminal makes an access to a server in an internal network, that is, the case where the mobile terminal is a call originating side, but it does not work well in applications where the mobile terminal can be a call terminating side such as Internet telephone and electronic conference system. Namely, in such applications, it is difficult for the other machines to ascertain the IP address currently used by the mobile terminal so that it is practically impossible to make an access to the mobile terminal from the other machines.
The Mobile IP is a scheme developed in order to resolve such a problem. This is a technique to make the mobile terminal appear as if it is located at its normal location with respect to the other machines regardless of its actual location on the network. This scheme has been put in the stage of RFC (Request For Comments) by the IETF (Internet Engineering Task Force) in October, 1996.
The operation of the Mobile IP will now be described with reference to FIG. 1. First, a home agent (HA: 10.1) is provided in advance at a network to which the mobile terminal (MH: 10.2) is normally connected. Then, when the mobile terminal (MH: 10.2) moves and is connected to a visited site network (FA: 20.1), it notifies the IP address (FA: 20.1) of the current location to the home agent (HA: 10.1). Thereafter, the home agent transfers an IP packet in which the IP address (MH: 10.2) of the normal location of the mobile terminal is specified as a destination to the current location of the mobile terminal by encapsulating that IP packet.
In the case of returning a message from the current location to an access source, an IP packet is outputted directly to the access source without using the home agent. In this manner, it becomes possible for each node on the network to make an access to the mobile terminal by using the home address of the mobile terminal regardless of where the mobile terminal is currently connected to.
The Mobile IP has been designed with an aim of requiring as little change in the existing Internet as possible so that it has a high affinity with the existing Internet but it is also associated with the following problems.
The first problem is that HA must always relay packets to MH. Consequently, even when HA is located very far away, it is necessary to transfer packets via HA and therefore there is a redundancy in the transfer route and the delay time of communication will inevitably increases.
The second problem is that packets destined to MH cannot be relayed by the usual routing control because the IP address of HA is held as it is even at a visited site and therefore there is an IP address in violation of the subnet model.
The third problem is that it is impossible to carry out the processing of a relay router with respect to a header of an original IP packet because a packet is transferred in an encapsulated form.
The fourth problem is that all communications with MH become impossible when HA malfunctions, because all communications must go through HA.
Among these problems, the first problem in particular is problematic in realizing real time communications even in Internet type network. For instance, an exemplary case shown in FIG. 2 has not only a considerable route redundancy but also a further delay time increase due to the bottleneck of the software processing because of the use of routers with conventional software processing, compared with the ideal case shown in FIG. 4.
For this reason, when the cut-through transfer is carried out by using the above described high speed router device, the transfer route becomes as shown in FIG. 3, and it can be expected that the delay time is considerably shortened compared with the case of FIG. 2. however, even when the Mobile IP technique and the high speed router device are used together, the problem of transfer route redundancy remains unresolved, and the delay time is still considerably large compared with the ideal case of FIG. 4.
Moreover, the Mobile IP scheme is a technique for accesses to/from mobile terminals in Internet type network and its goal is to enable Internet access from any visited site. For this reason, the Mobile IP scheme does not sufficiently account for the case of communication during moving, and adopts a method for making a re-connection when the communication is disconnected suddenly at a visited site. As a result, it requires a considerable amount of time in the handoff control associated with the moving, and it is difficult to realize the real time communications while moving as realized in the telecommunication type mobile communication system by the currently available technique. Consequently, there is a problem that it is difficult to realize the real time communications like Internet telephone under the mobile Internet environment.
Furthermore, the conventional mobile access technique such as the Mobile IP scheme, the mobility is supported by the routing at the network layer level so that the IP forwarding processing must be executed at a router device, and there is a problem that the router can be a bottleneck in the case where many mobile terminals coexist within a subnet of some router.